1. Field of the Invention
The present invention is directed to a device and method of deriving average flow velocity in a pipe carrying flow by using flow generated sound waves moving along and opposite to the flow. Flow information is obtained from the phase difference of sound signals sensed on the walls of the pipe.
2. Description of the Prior Art
Flow of gases and liquids are commonly measured by orifice flowmeters which produce a pressure difference varying as the square of the flow velocity. As the orifice wears out, the flow coefficient changes which requires either a replacement or recalibration. Flowmeters based on orifice plates require large upstream and downstream segments of straight pipe for proper operation. Accuracy suffers at low flow rates because the pressure difference decreases rapidly as the square of flow velocity. The minimum flow velocity is at best only on a quarter of the maximum flow velocity.
Vortex shedding flowmeters use the measurement of frequency of vortices shed periodically by a body projecting into the flow. In this case, the frequency varies linearly with flow over the range when eddy shedding occurs. Near zero flow, this flow meter will not work.
Acoustic flowmeters of the type suitable for flows in large pipes have been described. For example, the long wavelength acoustic flowmeter by James E. Potzick.sup.1 and Baldwin Robertson (U.S. Pat. No. 4,445,389; May 1, 1984) makes use of acoustic waves generated by loud speakers mounted on the side of a pipe. The waves are detected by two flush mounted microphones. Frequencies of operation of the source are adjusted such that maxima of sound pressure occur at the higher frequency which is exactly twice the lower frequency. At this frequency, phases are measured and interpreted. However, this type of acoustic long wave flowmeter can not be used for hot steam flows because a loud speaker can not be designed to operate in such hot flows. Heat and flow surges will destroy the loud speaker in a short time.
A flowmeter using the frequency of oscillations generated by a flow past a the cavity are chosen such as to provide a dominant frequency, pressure fluctuations sensed in the cavity are directly related to the flow past the cavity. However, extensive tests by us on resonant cavity oscillations have shown that when acoustic resonance occurs, the frequency becomes independent of flow.sup.3,4. When the cavity is not resonant, frequencies are proportional to flow but the constant of proportionality is not very constant. It is doubtful that an accurate flowmeter could be made using Fritz's concept.
Migliorini.sup.5 describes linear differential pressure flow metering with genuine 100:1 turndown capability which involves flow through an annular area which varies with flow. This uses spring loaded center bodies which move as flow increases thereby reducing the pressure difference.
Acoustic flowmeters using ultrasonic waves are used for liquid flows, mostly in biomedical applications. These generally use piezoelectric transducers which do not work at high temperatures. Also, ultrasonic waves cannot be generated efficiently in gaseous media by such transducers.